RadioBanter - View Single Post - Horizontal Dipole

Roy Lewallen · June 21st 09, 01:33 AM posted to rec.radio.amateur.antenna

wrote:
Frank wrote:

NEC will calculate "Space wave plus surface wave" if required.

Frank
Have you tried doing this calculation with a horizontally polarized VHF
antenna? What did you find?

Roy Lewallen, W7EL

I'm not sure what you mean.

My comment was directed to Frank. I'm hoping he'll try running the
calculation, or has already. He'll discover that:

1. A horizontally polarized wave does not propagate via a surface wave, and
2. The attenuation of surface waves increases with frequency. At VHF,
it's essentially zero at any useful distance.

Either of these is adequate to explain the result he'll see, or has
seen, that the result with surface wave is the same as the result
without surface wave, for this situation.

Richard and Dave have answered your question -- the reason for the zero
field at zero elevation angle EZNEC result is that the model ground is
perfectly flat and infinite in extent, and the observation point is
essentially at an infinite distance. This is useful for evaluating sky
wave propagation at long distances, but not for line-of-sight
propagation where Earth curvature can be a factor. Free space analysis
is better for this. Programs which calculate reflection coefficients for
various ground shapes aren't very useful at VHF and above, because local
features, especially in an urban environment, cause reflections that are
often much more significant than the ones calculated by the program. All
you can do with these programs in a situation like yours is to get a
general idea of what's happening.

EZNEC seems to say that a horizontally
polarized dipole seems to have zero gain (-99.99DBi) at zero degrees
elevation regardless of the frequency. So far, I have only tried 14
(the 20 meter example that came with EZNEC) Mhz, 491 Mhz (TV channel
17 center), and 527 MHz (TV channel 23 center). I switched to 527
because I can actually see a channel 23 transmitting antenna from my
window. For those who may not missed my original post, I find it hard
to believe a horizontal dipole tuned to the right frequency (near 1:1
SWR with 75 ohm source) would not be able to hear a signal coming from
zero degrees elevation. In the real world, there are all sorts of
reflections off of all sorts of things that will make it work, but is
it true that there should be no signal if everything was ideal?

If by ideal you mean that the ground is perfectly flat and infinite in
extent, yes.

Richard explained the attenuation of the E-field. That makes sense to
me, but doesn't really explain the other nulls at 6 degrees elevation
and every 6 degrees above that. There are strong positive lobes at 3
degrees and every 6 above that. The plot looks like a nice flower :-)
I would think that attentuation of the E-Field would explain zero
degrees, but as elevation increased, the attenuation would decrease.
The EZNEC plot looks more like it is showing additive and subtractive
combining of the signal.

That's exactly what EZNEC is doing. It calculates the sum of two "rays"
-- idealized straight line radiation from the antenna. One is the
"direct ray", which goes directly from the antenna to the observation
point. (Actually, the fields from all segments are individually
calculated and summed.) The second is the "reflected ray", which
reflects from the ground plane to arrive at the same distant point. Some
geometric investigation will show that the reflected ray at an elevation
angle of -x degrees will add to the direct ray at an elevation angle of
+x degrees at the distant point. The reflection coefficient of the
ground, which is determined by the frequency, wave polarization, and
ground conductivity and permittivity, is used to determine the amplitude
and phase of the reflected ray. The reflection coefficient of low angle
horizontally polarized waves is nearly -1 for any reasonable ground
characteristics, so the low angle pattern is nearly the same for a
horizontally polarized antenna over the Earth as for one over a
perfectly conducting ground. If you look the the distance from the
antenna to a distant point via the direct route and compare it to the
distance to the same point via the reflected route, you'll see that the
difference between the two routes (that is, the distances traversed by
the two "rays") monotonically increases with increasing elevation angle.
At zero elevation angle, the two distances are the same. Since the
reflected ray undergoes a phase reversal (as Richard explained),
expressed as a -1 reflection coefficient, the direct and reflected rays
cancel at the distant point, resulting in zero field strength. As the
elevation angle increases, the difference between the two two ray paths
increases, resulting in imperfect cancellation. At some elevation angle,
assuming the antenna is high enough off the ground, the difference in
path lengths will be exactly a half wavelength. At this angle, the two
rays will reinforce at the distant point. At a higher angle, they'll be
exactly one wavelength different, and the rays will cancel again. And so
forth. The path distance difference goes from zero at zero elevation
angle to a maximum straight up of twice the antenna height above ground,
so the number of "flower petals" you see depends on the height, in
wavelengths, of the antenna above ground.

Another reply mentioned a different program
that calculated ground wave in addition to skywave. Maybe that is
what I am missing. I normally think of ground wave as why VLF, LF,
and MF signals travel further than line of sight, though. Does ground
wave have a significant effect at VHF/UHF?

No, it doesn't. Which is why I posted the query to Frank.

I'm still confused,
Pat, N8CQV

Hope this helps. There's additional information in the _ARRL Antenna
Book_ and other sources which explains how the patterns of antennas are
affected by ground.

Roy Lewallen, W7EL